Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Proved Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2018 MMcfe
Extractive Industries [Abstract]
Beginning proved undeveloped reserves at December 31, 2017	6,914,287
Undeveloped reserves transferred to developed	(1,804,643)
Revisions	608,252	[1]
Purchases/(sales)	(127,601)
Extension and discoveries	2,725,240
Ending proved undeveloped reserves at December 31, 2018	8,315,535

[1] Includes 379 Bcfe of proved undeveloped reserves dropped due to the five-year rule which can be included in our future proved reserves as these locations are added back to our five-year development plan.